1. Field of the Invention
The present invention relates to an eyepiece optical system and an image display apparatus, and particularly to an eyepiece optical system that permits a two-dimensional image displayed on an image display device such as an LCD (liquid crystal display) to be enlarged and projected on an observer""s pupil so as to permit the observer to observe a virtual image of the two-dimensional image, and to an image display apparatus such as an HMD (head mounted display) employing such an eyepiece optical system.
2. Description of the Prior Art
Conventionally, several eyepiece optical systems having a so-called pancake-like structure have been proposed that employ a reflective/transmissive surface to permit observation of an enlarged image of a flat two-dimensional image. For example, U.S. Pat. No. 3,443,858 discloses an eyepiece optical system having a flat reflective/transmissive surface and a reflective/transmissive surface concave to the pupil side. U.S. Pat. No. 5,517,366 and U.S. Pat. No. 5,768,039 disclose an eyepiece optical system having two reflective/transmissive surfaces highly concave to the pupil side. U.S. Pat. No. 5,654,828 discloses an optical system in which a pancake-like structure and a lens are combined together. On the other hand, in recent years, image display devices for displaying two-dimensional images have rapidly been coming to have increasingly high pixel densities. For example, many models of LCDs are being put into practical use that are of sizes much smaller than one inch but that nevertheless offer high resolution.
However, with the simple combination of reflective/transmissive surfaces as disclosed in U.S. Pat. No. 3,443,858, when a small image display device is used, it is difficult to obtain a wide angle of view while maintaining a sufficient eye relief. The combination of reflective/transmissive surfaces as disclosed in U.S. Pat. No. 5,517,366 and U.S. Pat. No. 5,768,039 offers a wide angle of view; however, in this construction, it is difficult to form a wave plate or a cholesteric liquid crystal surface on the reflective/transmissive surfaces in order to eliminate direct light by exploiting polarization, because both of the reflective/transmissive surfaces are formed as curved surfaces. In addition, because the surface at the pupil side end is concave to the pupil side, it is difficult to obtain a sufficient eye relief. In the case of the optical system proposed in U.S. Pat. No. 5,654,828, because a positive or weakly negative lens is arranged on the image display device side thereof, it is difficult to cope with a small image display device while keeping rays telecentric toward the image display device.
An object of the present invention is to provide an eye piece optical system that can cope with miniaturization of image display devices and that can display high-resolution images, and to provide an image display apparatus employing such an eyepiece optical system.
To achieve the above object, according to one aspect of the present invention, an eyepiece optical system for projecting an enlarged virtual image of a two-dimensional image on the observer""s pupil is provided with, from the observer""s pupil side: a first reflective/transmissive surface formed as a flat surface; a second reflective/transmissive surface so shaped as to be concave to the pupil side; and a negative-powered optical element having a negative power and arranged between the second reflective/transmissive surface and the two-dimensional image. Here the following condition is fulfilled:
xe2x88x921.0 less than fs/f2 less than xe2x88x920.1
where f2 represents a focal length of the negative-powered optical element; and fs represents a focal length of the entire eyepiece optical system.
According to another aspect of the present invention, an image display apparatus is provided with: an image display device for displaying a two-dimensional image; and an eyepiece optical system for projecting an enlarged virtual image of the two-dimensional image on the observer""s pupil, the eyepiece optical system comprising, from the observer""s pupil side: a first reflective/transmissive surface formed as a flat surface; a second reflective/transmissive surface so shaped as to be concave to the pupil side; and a negative-powered optical element having a negative power and arranged between the second reflective/transmissive surface and the two-dimensional image. Here the eyepiece optical system fulfills the following condition:
xe2x80x83xe2x88x921.0 less than fs/f2 less than xe2x88x920.1
where f2 represents a focal length of the negative-powered optical element; and fs represents a focal length of the entire eyepiece optical system.